An aircraft 10 is shown in FIG. 1. Such an aircraft has a number of fluid systems, including a fuel system 20. The fuel system 20 has fuel tanks 21, 22 and conduits 23 for transferring fuel around the aircraft. Conduits 23 supply fuel to the aircraft's engines 24. Conduits 23 also transfer fuel between fuel tanks 21, 22. Such fuel transfer between fuel tanks 21, 22 affects the weight distribution of the aircraft 10. Therefore, it is important to establish the fluid volume in each fuel tank 21, 22.
It is known to measure the fluid volume in the fuel tanks 21, 22, using depth gauges at various locations to establish the fuel volume.
Alternatively, or in addition to, measuring the fluid volume in each fuel tank 21, 22, it is known to measure the volumetric flow of fuel along the conduits 23 between fuel tanks 21, 22.
Known methods of measuring volumetric fluid flow through a conduit include use of differential pressure flow meters, pitot tubes flow meters, thermal flow meters, coriolis flow meters and mass flow meters.
Differential pressure flow meters measure the pressure drop over an obstruction or restriction along the flow path of fluid flow to be measured. Pressure readings are taken upstream and downstream of the obstruction or restriction. Using the determined pressure drop and Bernoulli's flow equation, the fluid flow is calculated.
The presence of an obstruction or restriction in the flow path to obtain a pressure drop reduces the efficiency of the flow circuit. Furthermore, the obstruction or restriction may trap contaminates, such as ice, and lead to the meter producing a false reading. The obstruction or restriction may also disrupt and change the fluid flow characteristics.
Pitot tube flow meters measure the fluid flow velocity by converting the kinetic energy of the flow into potential energy. However, this also requires an obstacle in the flow path. Such an arrangement is also not suitable in some fluid systems, such as fuel systems.
Thermal flow meters use the calorimetric principle for fluid flow measurement based on two temperature sensors in close contact with the fluid but thermally insulated from each other. However, an obstacle is present in the flow path. Furthermore, the use of a heated ignition source is not acceptable for some fluid systems, such as fuel systems.
Coriolis flow meters use the Coriolis effect to measure the mass moving through a section. The fluid to be measured passes through a U-shaped tube that is caused to vibrate in an angular harmonic oscillation. Due to the Coriolis forces, the tubes will deform and an additional vibration component will be added to the oscillation. This additional component causes a phase shift on some places of the tubes which can be measured with sensors. However, such an arrangement is not suitable for aircraft and requires moving parts.
U.S. Pat. No. 2,897,672 A describes a flow meter which determines fluid flow by measuring the fluid inertia of fluid exiting a jet nozzle into a pipe of increased diameter. However, such a metering arrangement is present in the fluid flow and has a significant step change in flow section as the fluid translates through the jet nozzle and so induces a significant pressure drop. Furthermore, the sensors are exposed to the fluid and the fluid flow is single directional, i.e. out of the jet nozzle.